1. Field of the Invention
This invention relates to vehicular target detection systems, and more particularly, to a method and apparatus for interference avoidance by changing the transmit frequency of a vehicular target detection system when interference is encountered.
2. Description of Related Art
There is a continuing need to increase the density of vehicles traveling the world's roadways, and simultaneously to improve the safety of highway vehicle operations by preventing highway vehicles from colliding with moving and stationary objects (such as roadside obstacles and other vehicles). One means for accomplishing these seemingly contradictory goals is to monitor environmental conditions, such as the relative speed, direction of travel, and distance between vehicles sharing the roadway, and to use such information to provide direct indications to the vehicle's operator of potential danger. It is becoming increasingly more common for vehicular engineers to consider the use of target detection systems, such as microwave radar systems, as the means to monitor such environmental conditions.
A number of vehicular target detection systems are currently known in the art. One such system is described in copending U.S. patent application, Ser. No. 07/695,951, entitled "Multi-Frequency Vehicular Radar System", and assigned to the assignee of the present invention. The radar system described therein transmit three time-multiplexed signals, two of which are used to determine the range of an object (target), and the third of which is used to determine the relative speed of the target. The upper and lower frequencies are separated from the center frequency by about 250 kHz with the center frequency being about 24.125 GHz. The system transmits the signals, and receives reflections from targets. By processing the reflected signal, the system determines the range and relative speed of the targets.
In another vehicular target detection system described in copending U.S. patent application, Ser. No. 930,066 entitled "Multi-Frequency Multi-Target Vehicular Radar System Using Digital Signal Processing" filed Aug. 14, 1992, assigned to the assignee of the present invention, the microwave transceiver generates and transmits a signal in which only two frequencies are time division multiplexed. The two frequency radar system uses a digital signal processor within the digital electronics section to determine the range and relative motion of targets based upon the difference between the frequency of the transmitted signal and the frequency of reflections of the transmitted signal subsequently received by the transceiver. The digital signal processor performs a series of Fast Fourier Transforms (FFT) on a digitized representation of the output of a radio frequency (RF) mixer. The analog output of the mixer is a signal having a frequency equal to the difference between the frequency of the transmitted signal as applied to one input of the RF mixer, and the summation of reflections of the transmitted signal after reflecting off targets in the environment, as well as various other signals which have been received by the transceiver, as applied to a second input to the RF mixer. Analyzing the results of the FFT allows a microprocessor within the digital electronics section to determine the range and relative motion of a multiplicity of targets. The microcontroller generates visual and audible warnings to the vehicle operator.
However, a significant problem which plagues both of these target detection systems (and in fact all target detection systems which rely upon receiving a reflection of a transmitted signal from a target) is interference with the transmitted signal and the reflections thereof, which may make detection of such reflections difficult or impossible. For example, microwave interference caused by a variety of sources of microwave energy, such as other vehicular radar systems, burglar alarms, automatic door openers, and speed-measuring radar systems can render a microwave vehicular target detection system inoperative for periods of time which can range from fractions of a second to minutes. Such an occurrence reduces the effectiveness of the vehicular target system in detecting moving or stationary objects, such as other vehicles and roadside obstacles.
Therefore, it is desirable to provide a vehicular target detection system that can cope with the presence of interference in the environment in which the target detection system is operating. The present invention provides a solution to this problem.